Method and apparatus for increasing the adiabatic efficiency of a centrifugal compressor

ABSTRACT

A method and apparatus for increasing the adiabatic efficiency of a centrifugal compressor is provided. The apparatus includes a soft material insert that is placed in the case of the centrifugal compressor such that the soft material faces the compressor impeller. This gap between the impeller and the soft material insert can be minimized to prevent air from escaping the compressor and thereby increasing the efficiency of the centrifugal compressor.

FIELD OF THE INVENTION

[0001] This application claims priority to provisional U.S. application Ser. No. 60/430,814, filed Dec. 4, 2002.

[0002] This invention relates to a centrifugal compressor that may be used with an internal combustion engine that acts to increase the density of air or air-fuel mixture delivered to the engine for combustion. More specifically, the invention provides a method and apparatus for increasing the adiabatic efficiency of the centrifugal compressor by minimizing the distances between a compressor impeller and the compressor housing. Furthermore, the supercharger minimizes the reverse flow of air of the centrifugal compressor to increase the adiabatic efficiency thereof.

BACKGROUND OF THE INVENTION

[0003] An internal combustion engine utilizes fuel and air under pressure to create an explosion, or controlled burn, in a combustion chamber to convert translational energy into rotational energy. This rotational energy may be applied to the drive wheels of a vehicle, the propeller of a boat, airplane or any rotary device.

[0004] A typical engine utilized to power a vehicle or boat, contains cylinders, where combustion occurs, a piston which translates up and down the cylinder and a crankshaft rotatively mounted to the pistons via a connecting rod. These engines also utilize valve train components which consists of one or more camshafts or “cams”, valve lifters and push rods, or cam followers and valve springs along with rocker arms to open and close the valves of the engine to allow air and fuel to enter and exhaust to exit the cylinders during combustion. Each cylinder is associated with at least one intake and one exhaust valve. The fuel and air mixture may be delivered to the cylinders via a carburetor that intakes air and disperses fuel in to a metered amount of air taken in by the carburetor and directs the air-fuel mixture into an intake manifold. The intake manifold has runners that lead to each intake valve above the cylinders. Another common fuel delivery system is fuel injection, which may be electronically or mechanically controlled. In an injection system, fuel is injected into an air column via a throttle body or injectors that may be located in the cylinder or the runners of the intake manifold.

[0005] During the up stroke of the piston in the cylinder, the intake valve opens to allow fuel and air to enter the combustion chamber. The air and fuel mixture is commonly referred to as the “charge.” Somewhere near the top of the up stroke, the intake and the exhaust valves are both closed and, in gasoline engines, a spark plug creates a spark to ignite the charge that has been compressed as the pistons travel upward. In a diesel engine, the compression of the charge is sufficient to ignite the mixture. This results in a high temperature explosion which forces the piston downward, called the “power stroke,” thereby translating this movement via a connection rod to rotate the crankshaft which, in turn, translates this angular motion to the wheels of the vehicle via a set of gears and shafts.

[0006] Near the bottom of the compression stroke, the exhaust valve opens to expel the burnt fuel mixture out of the cylinder. After the piston changes directions and begins the up stroke, the exhaust valve continues to remain open and thereby forcing any remaining spent or “end” gases out of the cylinder. However, during this same time, the intake valve begins to open to recharge the cylinder with fuel. It is not until the piston has started to travel upward that the exhaust valve closes. Thus, at various times during the compression cycle, both the intake and exhaust valves will be open and closed at the same time. The timing of the opening and closing of the valves is controlled by the physical design of the eccentric lobes on the camshaft.

[0007] During the end of the exhaust stroke, if the intake valve opens too early, some of the exhaust gases may be pushed back through the intake valve and pushed into the intake manifold instead of exiting the exhaust manifold. In an attempt to increase the efficiency of the combustion process, various valve train, exhaust and intake manifold designs have been developed. Conversely, less efficient designs often result in the expulsion of a portion of the air-fuel mixture that has not undergone combustion.

[0008] Various systems may be used to control the combustion process. A distributor and ignition coil may control the electrical system, which creates the electrical impulse delivered to the spark plug, mechanically. The distributor contains a rotor, which is controlled by a gear and shaft driven by the camshaft. Electronically controlled ignitions are also available which utilize a computer and coil to create the electrical impulse delivered to the sparkplug.

[0009] Engines also utilize various air intake devices. The most basic air intake system is a passive system that directs the external air or “atmospheric air” into a carburetor where gasoline is metered and dispersed from a bowl into the air. Beneath the carburetor is an intake manifold, which directs the charge to the cylinders via channels feeding the intake valves. A throttle body in a fuel injection system serves as the carburetor by injecting fuel into the air column, which is fed into the intake manifold and directed to the cylinders. Direct injection systems are also widely used. These systems may inject the fuel into the runners of the intake manifold or directly into the cylinder. Typically, these fuel injection systems utilize a computer to control when the fuel and how much is injected in to each cylinder or manifold runners.

[0010] As commonly known, oxygen is required to support the combustion of hydrocarbons. The amount of oxygen molecules in the charge at a given time depends upon the atmospheric conditions. On a humid day, a given volume of air will contain a specific percentage water molecules along with oxygen and other gas molecules. Conversely, on a less humid day more oxygen molecules will be present in the same volume of air and available for combustion. The electronic fuel injection systems can be programmed to adjust for changing atmospheric conditions by using an absolute manifold atmospheric pressure sensor commonly referred to as a “MAP sensor that is monitored by a computer or electronic control unit or “ECU.”

[0011] The ECU of a fuel injection system may utilize a single engine condition such as engine speed measured in revolutions per minute “RPM”, to reference a “look-up” table stored in memory of the system that contains the length of time to keep an injector open at that particular engine condition. Likewise, the ECU may utilize the intake manifold absolute pressure measured by the MAP sensor to determine the length of time to injection the fuel. This “length of time” is often referred to as “pulse width.” More complex systems may use complex algorithms to determine the injector pulse width based upon multiple engine conditions such as MAP, RPM, engine temperature, intake air temperature and other factors. These multiple engine conditions are monitored by the ECU, which computes the pulse width of each fuel injectors in an attempt to reach the most efficient, burning air-fuel ratio called the stoichiometric ratio of 14.7 to 1 (14.7:1) for gasoline engines. In a carbureted system, the adjust for a change in atmospheric conditions, the operator may size of the orifices or jets which are used to transfer the fuel from the fuel holding area of the carburetor, “bowls” to the air column. A disadvantage of most carbureted engines is that the jets must be changed manually and are typically not electronically controlled.

[0012] The intake manifold of an engine is designed to deliver equal quantities of the air-fuel charge to each cylinder while preventing the charge for one cylinder from interfering with the flow of the charge to another. Many factors influence the flow of charge to the individual cylinders including the number of cylinder such as in passenger vehicles 4 to 12 cylinders, their physical relationship such as a “V” style or straight arrangement of the cylinders, the depth and diameter of the cylinders, throttle position—wide open versus closed as in idling, the cross-sectional area of each runner which directs the charge to an individual cylinder and numerous others. The rise tracts leading to each cylinder must be designed to produce a minimum idling air velocity that can support the heavier fuel particles in the air stream while large enough to support combustion at wide open throttle. Likewise, the optimum manifold design for a low engine speeds may not be adequate for that of a high speed engine due the pressure-waves created in the manifold tracks and the opening and closing of the intake valve. Every time the intake valve opens, the pressure in the cylinder reduces thereby creating a negative pressure-wave, which travels through the air column to the atmosphere which influences the amount of charge delivered to the remaining cylinders. Similarly, as the speed of the engine increases, the length of time the intake valve is open to fill the cylinder decreases and thereby decreases the volumetric efficiency of the engine. Therefore, the goal of the manifold design is to create the large amount of volumetric efficiency for a particular engine configuration over the range of operating conditions from closed to wide open throttle.

[0013] One method of increasing the efficiency of an engine is to more efficiency use the energy contained in the fuel. In a typical gasoline engine, approximately thirty percent of the potential energy of the gasoline is converted to work. The remaining seventy percent is lost to heat energy in the form of friction, heat to the surrounding air and the engine's cooling system and heat in the outgoing exhaust gases. A turbocharger utilizes this wasted energy, heat, in the exhaust to drive a centrifugal compressor wheel that forces air into the manifold of the engine. A turbocharged engine routes the exhaust from the cylinders through the blades of a turbine which causes the compressor to spin. The compressor is open to the ambient air and when the exhaust turbine drives the turbine compressor, ambient air is drawn into the compressor and forced through an air cooler before entering the cylinders of the engine. Therefore, a turbocharger increases the volumetric efficiency of an engine by forcing more air into the cylinders.

[0014] However, a turbocharger does so at the expense of increasing the exhaust manifold exhaust back-pressure which inhibits the escape of the spent gases out of the cylinders. This increased pressure causes pre-detonation requiring and is typically overcome by lowering the engine compression ratio. Another disadvantage of turbochargers is that at low exhaust pressures or low engine speeds the exhaust is insufficient to drive the turbine wheel. This results in what is know as “turbo lag” which is eventually overcome as the engine speed increases. Turbo lag also results in lower engine efficiencies due to and the reduced compression ratio required for the turbocharged engine such that the turbocharged engine's output and fuel consumption is lower than naturally aspirated engines.

[0015] Under stop-and-go city driving conditions the turbocharger will often spool (spin) and unspool many times creating harsh physical conditions for the turbine wheels and components. To survive this spooling and unspooling, the dimensional clearances on the turbine and compressor wheel is fairly large to allow the turbines spool down freely. During these periods the turbine wheel may contact the turbine or compressor housings. Thus, the turbines and connecting shaft must be configured to undergo these harsh operating conditions.

[0016] A second method of increasing volumetric efficiency by increasing the density of the charge entering the cylinder is with a supercharger. A supercharger is an active air intake system that acts like an air pump to actively draw in air from the atmosphere, compress it and force the air into the cylinder by increasing the ambient pressure of the manifold. This increases the density of the charge by forces more air molecules into the combustion chamber and thereby increasing the amount of oxygen molecules available for combustion. Thus, the efficiency of the combustion process is increased along with power output and speed of the engine. Generally, as the engine speed increases, the pressure of the air delivered by the supercharger also increases and thereby compensates for the reduced time the combustion chamber has to intake the charge. Thus, the rate of power increases proportionally with the rise in engine speed.

[0017] Various designs of superchargers have been developed over the years for various engine applications. In fact, superchargers where first used on diesel train engines in the late 1800's and have been available on automobiles since the early 1900's. As such, various drive mechanisms have been developed to reduce the amount of horsepower loss created by use of power from the engine to operate the supercharger. Likewise, various mechanisms for compressing the air have also been developed. Examples of supercharger designs include the sliding vane or plane supercharger which utilizes a series of vanes positioned radially outward from a drum inside a casing. As the drum rotates in the casing, the charge enters the casing and as the drum rotates, it compress the charge by reduce the volume of the chamber before allowing the charge to exit the supercharger. The volumetric displacement of vane-type superchargers rises as the drum speed increases. However, the rise in efficiency drops as the boost pressure increases due to the backpressure leak past the vanes. Similarly, as the drum speed slows, there is more time for the charge to escape between the drum, vanes and casing. One disadvantage of the vane style supercharger is that it requires lubrication between the sliding vanes and the casing. Although the sliding vane style supercharge has it limitations, it can increase the pressure of the charge and increase the peak engine torque up to 30% in gasoline engines.

[0018] Another common and early design supercharger is the Roots rotary positive displacement supercharger, which is often referred to as a “blower.” The Roots brothers developed their blower while attempting to create a better water wheel for a mill in the late 1800's. Although they learned that it was inefficient for displacing water, they discovered that it was successful for pumping large volumes of air at low rotor speeds. Roots blower uses two rotors with typically helical lobes that rotate in opposite directions, one clockwise and one counterclockwise, at the same speed without touching each other. This type of supercharger creates pressure pulsations and turbulents, which generate loud sound waves. Besides the dispersion of energy in the form of sound waves, Roots blowers also greatly increase the temperature of the charge before the air-fuel mixtures enters the cylinder which has the overall effect of reducing the density of the delivered charge. One of the advantages of a Roots blower is that the rotating lobes do not make contact and therefore all the rotors operate dry without the need for an oil spray. A disadvantage of the Roots style blowers is that rotors turbulate the charge as the lobes of the rotors compress the charge, which creates heat. Although this heat raises the pressure of the charge, it does not increase the density and decreases the efficiency of the blower. Roots type blowers are often easy to identify by sound in that they create a distinct pulsating sound, especially when at idle.

[0019] A similar supercharger is the screw-type Sprintex supercharger that utilizes two screws, one male and one female to compress the charger. Like the Roots blower the lobes on the rotors or screws do not touch. The charge is compressed as it travels through the rotation of the screws and can achieve large percentage of volume compression above 60%.

[0020] Lastly, centrifugal supercharges increase the pressure of the intake air by utilizing a compressor powered by the engine. Even though Roots and screw-type superchargers are often referred to as “blowers”, they are more accurately described as positive displacement pumps. The output of the charge is directly proportional to the speed at which the rotors are rotating. On the other hand, a centrifugal supercharger is an inertial compressor that behaves much like a fan, or a true “blower.”

[0021] The centrifugal supercharger utilizes an impeller encased in a housing shaped with spiral land chambers like a snail's shell. An inlet hole is located at the center of the impeller and the outlet is at the end of spiral chamber (or volute).

[0022] Air enters the supercharger inlet and is rotated, advanced, radially displaced by the impeller, creating a low pressure that draws in more air into the inlet.

[0023] Air is rapidly accelerated radially towards the outer tips of the rotating impeller by the centrifural force. The impeller size and design is a critical component of the centrifugal supercharger. Overall, the centrifugal supercharger has a much smaller physical size than the vane, Roots or screw-type supercharger. Moreover, the centrifugal supercharger is also more versatile because it can be positioned almost anywhere in on the engine within reach of a drive belt. The vane, Roots and screw-type superchargers are typically placed directly above the intake manifold to force the compressed charge into the cylinders.

[0024] Another advantage of centrifugal superchargers is its adiabatic efficiency.

[0025] Due to the fact the centrifugal supercharger does relatively little redundant work on the air, the temperature of the charge rises minimally. The weight of the air molecules and the rotating impeller are used to squeeze the charge and increase its density. The air actually compresses as it enters the inlet and is continuously squeezed and forced outward towards the volute. Thus, the kinetic energy of [he molecules is efficiently converted to potential energy in the form of pressure while increasing the density of the charge as the fuel is delivered to the compressed air with little energy loss due to the fact that only a small amount of energy was converted into heat. It is not uncommon for even the simplest of centrifugal superchargers to reach 70 to 80% adiabatic efficiency.

[0026] Another advantage of the centrifugal supercharger is its reliability due to the fact that only the impeller rotates within the housing and does not come in contact with the casing. As such, the need for lubrication in the casing as seen with the vane supercharger is not needed. Another distinguishing characteristic of the centrifugal supercharger is that a bypass valve system can be utilized. At idle or during deceleration conditions of high RPM and low air demand, the charge output of the centrifugal supercharger may be much greater than what the engine can consume and create a pressure rise against the throttle valve. The air delivered is greater than that necessary to support combustion at idle. A bypass can be utilized to direct the excess air pressure back to the intake side of the supercharger or the atmosphere.

[0027] The centrifugal supercharger is driven by the crankshaft via a belt and therefore, as engine speeds increase, the impeller speed also increases. This creates an approximately linear relationship between boost and engine speed.

[0028] However, the impeller of centrifugal supercharger typically rotates between 6 to 12 times the engine speeds. The power loss from driving a centrifugal supercharger is less than that that of the Roots style and vane supercharger configurations. Based upon approximate flow characteristics, a Roots style blower being driving at 4,000 revolutions per minute will consume approximately 50% of the gross energy to drive the blower. Conversely, the centrifugal supercharge at the same speed will consume approximately 30% of the gross energy. Therefore, although the Roots and sliding vane style superchargers may create more boost at a common volumetric airflow, the centrifugal supercharger is more efficient making it an ideal supercharger for a multitude of engines.

[0029] This proportional relationship makes the centrifugal supercharger ideal for passenger vehicles and light truck applications. At low engine speeds, the centrifugal supercharger practically doubles the horsepower of an engine. At higher engine speeds, the supercharger continues to increase the engine's horsepower, but only at a lower percentage. The resulting effect is an overall increase in the average horsepower of the engine over a range of driving conditions. In turn, the engine may become more volumetrically efficient and therefore, increases fuel mileage. Likewise, along with increased fuel economy, the centrifugal supercharger also lowers emissions by providing more oxygen to each cylinder to support more efficient combustion so that nearly all combustibles are consumed by the engine and do not leave the engine as exhaust). Under cruise conditions, (i.e., traveling at an average speed with out the need increase power output to maintain speed) the centrifugal supercharger has little effect on the engine and requires negligible mechanical energy to remove itself from the engine system.

[0030] Various improvements have been made to centrifugal superchargers in an attempt to increase its overall efficiency. Components that create friction and consume engine such as the transmission and bearing systems have optimized to reduce energy losses. Various impeller configurations have been developed for greater control of blade and air speeds. One known method of increasing the adiabatic efficiency of a centrifugal supercharger is to minimize the gap between the impeller blade and the housing. This decreases the amount of air that escapes backward out the inlet of the supercharger, often referred to as “back slippage.” In practice, however, this is extremely difficult. The gap must be maintained as the impeller accelerates and decelerates. If the impeller touches the housing, great force is place on the impeller shaft that is transferred to the transmission. This “shocking” of the impeller shaft often results in bearing, shaft, or impeller failure. The impeller is typically composed of a material that is softer than that of the case. If the impeller were to touch the case, the tips of the blades deform or fail. Small fragments of material may enter the engine intake and harm the engine. Another consideration is minimizing the gap between the impeller and the housing is the smaller the gap the greater the frictional heating created by the air molecules and the impeller blades.

[0031] To overcome these problems, the distance between the impeller and the housing may be varied along tips of the impeller blade. However, a gap must always remain while the impeller is spinning to prevent catastrophic failure of the centrifugal supercharger system. Therefore, a method of minimizing the gap between the tips of impeller and the housing while reducing the added frictional heat is desired.

[0032] Likewise, in a turbocharger, the smaller the gap between the compressor impeller and housing also increases the adiabatic efficiency of the turbocharger. However, turbochargers are typically designed such that the blades of the impeller may contact the housing as the turbocharger spools up and down. Therefore, decreasing the gap may result in failure of the compressor impeller or housing if it excessive. Thus, a method of decreasing the gap between the impeller and the housing with minimal failures is also desired.

[0033] Previous attempts at solving these problems were attempted by using a Teflon or similar material, which was sprayed on inner surface of the case in an attempt to minimize the distance between inner casing wall and the impeller. Although, the sprayed insert was some what capable of reducing the amount of back slippage of air. However, if impeller 50 struck the sprayed soft material insert, the adhesion bond of the sprayed material would fail if the impeller scrapped the inner casing wall. The bond of the sprayed material would fail and the entire coating would be removed. Likewise, this method did not reduce the shocking loads on the shaft of the impeller due to the effective hardness of the sprayed coating was that of the backing material—the case. The sprayed material did not have the necessary physical properties to absorb the mechanical shock waves created by touch down. To overcome these limitations, complex materials of varying hardness have been attempted, but failed to remain bonded in the casing or were too hard such that if the impeller scrapped the material on the wall casing, the impeller or the shaft and bearing systems would fail. Thus, a method to overcome these limitations is desired.

[0034] Lastly, the adiabatic efficiency of all centrifugal compressors operating on compressible fluids such a turbine engines and radial fans can be increased by reducing the distance between the impeller or turbine blades and the compressor housing. However, the touching or excessive touching of the impeller blades to the housings may result in catastrophic failure. This is especially true of jet turbine engines where the blades grow or elongate as they absorb heat. Thus, a method of increasing the gap between the impeller and housing while preventing or minimizing the effects of the impeller toughing the blade to the housing is also desired.

BRIEF SUMMARY OF THE INVENTION

[0035] In accordance with these and other objects evident from the following description of the preferred embodiments, the present invention provides a method and apparatus for increasing the adiabatic efficiency of a centrifugal compressor by reducing the amount of air that escapes back through the compressor by using a soft material insert in the housing or case of the compressor. This reduced “back slippage” thereby increasing the adiabatic efficiency of the centrifugal compressor.

[0036] One embodiment utilizes a soft material insert having a reinforcing material transversing the inner portion of the compressor case held by mechanical fasteners.

[0037] A second embodiment minimizes the distance between the impeller and the case using the soft material insert held by multiple retainers.

[0038] A third embodiment of the present invention minimized the distance between the impeller and the compressor housing utilizing a soft material having a reinforcing material positioned next to the interior surface of the wall of the case.

[0039] A forth embodiment minimizes the distance using a soft material insert to reduce the distance between the impeller and the case at the axial compressor portion of the case held by interference fit.

[0040] A fifth embodiment utilizes a soft-material insert at the axially portion of the case to reduce the amount of air which escapes out of the intake of the supercharger.

[0041] Other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments and the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a top plan view schematic representative of an internal combustion engine including a centrifugal supercharger;

[0043]FIG. 2 is a cross-sectional view of a centrifugal supercharger taken along line 2-2 of FIG. 1 showing an intake air inlet, compressor chamber, impeller shaft, volute portion and supercharger casing;

[0044]FIG. 3 is a cross-sectional view of centrifugal supercharger taken along line 2-2 of FIG. 1 showing a toroid insert in the axially portion of the compressor and a retaining ring to hold secure the toroid insert;

[0045]FIG. 4 is a portion of a cross-sectional view of the centrifugal supercharger taken along line 2-2 of FIG. 1 illustrating an alternative embodiment toroid insert having a crenulated surface;

[0046]FIG. 5 is a cross-sectional view of the centrifugal supercharger taken along line 2-2 of FIG. 1 showing a large surface compressor wall insert held by torus retainers;

[0047]FIG. 6 is a portion of a cross-sectional view of the centrifugal supercharger taken along line 2-2 of FIG. 1 illustrating a smooth, large surface compressor wall insert held by torus retainers;

[0048]FIG. 7 is a portion of a cross-sectional view of the centrifugal supercharger taken along line 2-2 of FIG. 1 illustrating a crenulated, large surface compressor wall insert held by torus retainers;

[0049]FIG. 8 is a cross-sectional view of the centrifugal supercharger taken along line 2-2 of FIG. 1 having a large surface insert held having cylindrical retaining tabs that are force-fitted into the centrifugal supercharger case;

[0050]FIG. 9 is cross-sectional view of a centrifugal supercharger taken along line 9-9 of FIG. 1 illustrating cylindrical receiving holes in the interior of the supercharger case;

[0051]FIG. 10 is a cross-sectional view of the centrifugal supercharger take along line 2-2 of FIG. 1 showing a entire surface insert having a reinforcing material and mechanical fasteners; and

[0052]FIG. 11 is a broke-away view of area 11 shown in FIG. 10 showing the outer layer of the entire surface insert and the reinforcing material beneath the insert.

DETAILED DESCRIPTION OF THE INVENTION

[0053] The invention may be embodied in various forms; however, the invention is described with respect to the following embodiments.

[0054] Turning initially to FIG. 1, a centrifugal supercharger in the form of a supercharger 20 is selected for illustration is shown for use with an internal combustion engine 22 of a vehicle such as a boat or automobile. Other centrifugal compressor such as turbochargers, turbine engines, centrifugal pumps operating on compressible fluids also benefit from the disclosed invention. Although the illustrated engine 22 is a “V” style engine such one have four cylinders on one side and another four opposing cylinders on the opposite side, the principles of the present invention are equally applicable to various other types of engines. It is noted, however, that supercharger 20 is preferably driven directly by engine 22, with a crankshaft 24 and a belt drive 26 providing driving power to supercharger 20. Moreover, supercharger 20 is connected to an engine intake 28 (e.g., an intake plenum box, intake manifold, etc.) by a conduit 30, such that pressurized air generated by supercharger 20 is directed to intake 28. Again, the principles of the present invention are not limited to the illustrated application, but rather inventive supercharger 20 may be associated with any system in which a highly pressurized air stream is desired. For example, it is entirely within the ambit of the present invention to utilize supercharger 20 with various other types of internal combustion engines.

[0055] Supercharger 20 includes a case 32 that defines compressor and transmission chambers as identified hereinbelow. As illustrated in FIG. 1, case 32 generally includes three main sections, 34, 36 and 38 that are formed of any suitable material (e.g., polished cast steel) and interconnected as will be described.

[0056] Case sections 34 and 36 cooperate to define a compressor chamber 40 in which incoming fluid (e.g., air, air-fuel mixture, etc.) is pressurized and accelerated (see FIG. 2). Case section 34 presents a central inlet opening 42 (see FIG. 4) through which fluid enters chambers 40. A filter 41 (see FIG. 1) is preferably provided at inlet opening 42, as shown, or somewhere upstream from opening 42. Although not illustrated, inlet opening 42 may alternatively communicate with a forward open conduit (not shown) that extends toward the front of the powered vehicle, such that airflow to supercharger 20 is facilitated when the vehicle is moving in a forward direction. Case section 34 is configured in such a manner that a portion 44 of compression chamber 40 extends circumferentially around inlet opening 42 to form a volute of progressively increasing diameter. Volute portion 44 of compressor chamber 40 terminates at a tangential outlet opening 46 (see FIGS. 2), with the later communicating with engine intake 28 via conduit 30 (see also FIG. 1). In this regard, fluid entering compressor chamber 40 flows axially through inlet opening 42, is propelled generally radially into volute portion 44, and then directed along a generally circular path to outlet opening 46.

[0057] As shown in FIG. 2, inside and separate of case 34 is a rotatable impeller 50 which is coupled to a transmission and power source (not shown) via an impeller shaft 52 to rotate the impeller held to the impeller by a screw (not shown) beneath cap 54. Case 34 contains an inner casing wall 56 that faces impeller 50 and forms a cylinder which extends downward into a bell-like form as shown in FIG. 2. Case 34 has a radial compressor portion 58 located in compressor chamber 40 that is located above a bell-shaped radial compressor portion 60 of case 34 as seen in FIG. 2.

[0058] Intake air enters centrifugal supercharger 20 through intake inlet opening 42 and enters compressor chamber 40. As impeller 50 rotates, air molecules are forced to the outer casing due to centrifugal acceleration. At the axial compressor portion 58, the amount of work done on air molecules is relatively small and the amount of friction resulting in heat is relatively small. As the air molecules continue down impeller 50 to radial compressor portion 60, the amount of work performed on the molecules is much greater. Here the molecules have been forced together creating greater pressures and heat due to friction. As the air molecules travel through compressor chamber 40, some air molecules are accelerated in the backwards direction and escape through intake inlet opening 42. These escaping air molecules are a loss to the system. Therefore, the adiabatic efficiency of centrifugal supercharger 20 can be improved by preventing the loss of the escaping air molecules and by reducing the amount of energy lost as heat due to friction of the air molecules on impeller 50.

[0059] One method of improving the adiabatic efficiency of centrifugal supercharger 20 is to minimize the distance between inner casing wall 56 and impeller 50. However, simply minimizing this distance across along impeller 50 creates significant problems. Minimizing this distance creates friction in the system as many air molecules enter intake inlet opening 42, bump into each other and impeller 50 creating friction that heats the air.

[0060] Another concern about decreasing the distance between impeller 50 and inner casing wall 56 is the possibility of impeller 50 touching inner casing wall 56. This event is known as “touch down.” In some centrifugal compressors such as a turbocharger, this event is common when impeller 50 of the turbocharger compressor spools up and is allowed to suddenly spool down which may cause impeller 50 to strike inner casing wall 56. However, in other centrifugal compressors such as most centrifugal superchargers and jet engines, the touching of impeller 50 to inner casing wall 56 often leads to catastrophic failure of the centrifugal compressor. Touch down in these systems results in great shocking loads to impeller shaft 52 and the bearing systems in the transmission and power source which may destroy either impeller shaft 52, the bearings or transmission or a combination thereof. Impeller shaft 52 may fail or impeller 50 and fragments of the components may enter engine intake 28 and significantly damage or lead to catastrophic failure of internal combustion engine 22 and failure of centrifugal compressor 20.

[0061] To overcome these problems, a soft material insert 62 has been developed. The soft material insert 62 may be a variety of materials having a material hardness softer than that of impeller 50. In one embodiment of centrifugal supercharger 20, impeller 50 is preferably machined from 7075 T-6 aircraft aluminum billet, although other suitable materials (e.g., cast aluminum, aluminum coated with a ceramic coating, etc.) may be used. It is further preferred to use the impeller commercially available from the assignee of record of the invention claimed herein. A preferable soft material is Teflon having a tensile strength between approximately 2,000 and 4,500 psi, which may be machined or molded into the configuration preferred for the present invention. However, other soft materials which have a hardness less than that of rotatable impeller 50 may be used such a soft polyethylene plastics, polyester based materials.

[0062] Turning to FIG. 3, one embodiment of the invention uses a soft material insert 62 having a toroid shape that is placed in axial compressor portion a radial compressor portion of case 58. Inner casing wall 56 has been machined to create an annular notch 64 for receiving toroid-shaped soft material insert 62. Toroid-shaped soft material insert 62 is held in place using a retaining ring 66 preferably composed of aluminum or similar material. Retaining ring 66 holds toroid-shaped soft material insert 62 in radial compression. As an alternatively, toroid-shaped soft material insert 62 made also be adhesively bonded to the surface of axial compressor portion 58. Likewise, the surface or axial compressor portion 58 may be gnurled before inserting toroid-shaped soft material insert 62. The inner surfaces of retaining ring 66 and soft material insert 62 are machined to form a smooth transition surface that follows the contour of inner casing wall 56 of casing 34. In the preferred embodiment, a toroid-shaped soft material insert 62 is placed in axial compressor portion of case 58 as shown in FIG. 3. At axial compressor portion of case 58, the distance between impeller 50 and axial compressor portion of case 58 can be brought to a minimum to prevent the air molecules that have entered compressor chamber 40 from escaping out intake inlet opening 42, back slippage. Moreover, toroid-shaped soft material insert 62 can be placed along any section of axial compressor portion 60 to further direct airflow or to minimize or maximize the area of cover with toroid-shaped soft material insert 62. Likewise, toroid-shaped soft material insert 62 at axial compressor portion radial compressor portion of case 58 can minimize the escape of air molecules at the portion of compressor chamber 40 where minimal work has been done on the air molecules such that the air molecules are “coldest” at this location. Thus, minimal frictional heating is created in this portion of compressor chamber 40.

[0063] If impeller 50 where to touch inner casing wall 56 in this preferred embodiment, impeller 50 would touch toroid-shaped soft material insert 62. Due to the fact that the hardness of preferred aluminum impeller 50 is greater than that of toroid-shaped soft material insert 62, impeller 50 will slightly remove or shave a portion of soft material insert 62 without creating shocking forces great enough to damage impeller shaft 52 or the bearing systems in the transmission (not shown). Moreover, any fragments or shavings of soft material insert 62 that enter the combustion chamber of internal combustion engine 22 are consumed during the combustion process without harming internal combustion engine 22.

[0064] An alterative preferred embodiment utilizing toroid-shaped soft material insert 62 at axial compressor portion of case 58 is shown in FIG. 4. The surface of toroid-shaped soft material insert 62 that faces impeller 50 has been crenulated such that the surface has barbs 68. In one embodiment, barbs 68 extend outwards towards impeller 50 by 0.033 inch. Barbs 68 are used to minimize the distance between impeller 50 and axial compressor portion of case 58 and reduces the surface area that impeller 50 can contact inner casing wall 56. Actual dynameters testing of internal combustion engine 22 with centrifugal supercharger 20 having soft material insert 62 with the crenulated surface has shown increased horsepower (approximately 5 to 20 horsepower) over the same centrifugal supercharger without toroid-shaped soft material insert 62.

[0065] A third alternative embodiment utilizing toroid-shaped soft material insert 62 is shown in FIG. 5. In this embodiment, toroid-shaped soft material insert 62 transverses both the axial compressor portion of case 58 and radial compressor portion 60 in one continuous piece. The area covered by toroid-shaped soft material insert may be varied over the inner wall 56 to provide the proper air flow, cooling property or producibility. As seen above, inner casing wall 56 has been machined to receive toroid-shaped soft material insert 62 such that the contour of inner casing wall 56 is maintained. Multiple torus retaining rings 70 are used to hold soft material insert 62 (see FIG. 6). The surface of soft material insert 62 that faces impeller 50 has been channeled to receive torus retaining rings 70 such that each torus retaining ring 70 is sunk below the surface. As such, if impeller 50 (not shown) makes contact with toroid-shaved soft material insert 62 with torus retaining rings 70, impeller 50 will touch toroid-shaped soft material insert 62 and not torus retaining ring 70. Toroid-shaped soft material insert 62 is held in place using retaining channel 71 to secure toroid-shaped soft material insert 62 in radial compressor portion 60 of case 32. As seen above, alternatively, the surface of toroid-shaped soft material insert 62 may also be crenulated as shown in FIG. 7 such that barbs 68 are present on the surface facing impeller 50. Alternatively, toroid-shaped soft material insert 62 may be further secured using adhesive to bond toroid-shaped soft material insert 62 to the inner wall 56 of case 32. Likewise, the surface of inner wall 56 or the mating surface of toroid-shaped soft material insert 62 may be gnurled to provide a better gripping surface. Moreover, toroid-shaped soft material insert 62 may be adhesively bonded to the inner wall 56 or used as a reinforcement to torus retaining rings 70

[0066] An advantage of toroid-shaped soft material insert 62 extending over both axial compressor portion 58 and radial compressor portion of case 60 is that the distance between the toroid-shaped soft material insert 62 and impeller 50 may be varied over the entire surface. For example, diesel internal combustion engines typically operate at lower engine speeds than gasoline engines. When centrifugal supercharger 20 is used on a diesel engine, the impeller speeds (10 to 2000 revolutions per minute, RPM) will be much lower than that of a gasoline engine (1000 to 10,000 RPM). At lower impeller speeds, it may be advantageous to minimize the distance between toroid-shaped soft material insert 62 and axial compressor portion of case 58 and also minimize the distance at a portion of radial compressor portion of case 60.

[0067] Another embodiment of toroid-shaped, large soft material insert 62 covering axial compressor portion of case 58 and radial compressor portion of case 60 is shown in FIG. 8. This configuration utilizes cylindrical locating tabs 72 that protrude on the case side of soft material insert 62. As shown in FIG. 9, the interior of case 34 has been modified by machining receiving holes 74 for receiving locator tabs 74. For assembly, toroid-shaped soft material insert 62 is force fit into case 34 such that locating tabs 72 are force fit into and held by receiving holes 74. Alternatively, cylindrical locating tabs 72 may be replaced with protruding annular rings (not shown). In this alternative, case 34 contains annular channels (not shown) which the receive protruding annular rings (not shown). Both embodiments may also utilize adhesives to secure toroid-shaped soft material insert 62 to inner wall 56 of case 34 and gnurling of the mating surfaces.

[0068] Lastly, another alternative embodiment is shown in FIG. 10 which utilizes a soft material insert 62 that extends along the entire surface of the inner casing 34 to cover the portion of the inner casing wall 56 located above the axial compressor portion of case 58. This large area soft material insert 62 is further supported by a reinforcement material 76 sandwiched between inner casing wall 56. Reinforcement material 74 is composed of aluminum, steel, ceramic, composite or similar materials (see also FIG. 11) and is bonded to soft material insert 62 using a high strength epoxy or similar bonding agent. To secure reinforcement material 74 and soft material insert 62, an anchor nut is placed in reinforcement material 74 and soft material insert 62. A threaded fastener 80 is inserted through case 34 and is received by anchor nut 78. An annular retaining ring 82 is received by a notch 84 in soft material portion 62 and holds reinforcement material 74. A mechanical fastener 86 penetrates annular retaining ring 82 and is received by a threaded portion in case 32. Soft material insert 62 with reinforcement material 74 is held in axial compression at mechanical fastener 80 and radial compression at mechanical fastener 86. Alternatively, soft material insert 62 and reinforcement material 72 may be adhesively bonded to inner casing wall 56. However, to ensure the bond, mechanical fasteners 80 or 82 or both are used.

[0069] Another advantage of larger material insert 62 as shown in FIG. 5 through 10, is that the large mass of the soft material insert 62 also the friction heat to lower the temperature of the air molecules corresponding to the intake charge sent to the cylinders. The cooler the intake charge, the denser the charge, the in more oxygen molecules for combustion, which results in more horsepower produced by internal combustion engine 22.

[0070] While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims. 

I claim:
 1. A method of increasing the adiabatic efficiency of a centrifugal compressor having a case with an axial compressor portion and a radial compressor portion, a rotatable impeller, an impeller shaft drivingly coupled between the impeller and a power source, the centrifugal compressor acting upon a compressible fluid, the method comprising: minimizing the distance between an impeller and an interior surface of a wall of the case.
 2. The method of claim 1 wherein the step of minimizing the distance between the impeller and the interior surface of the wall of the case includes inserting a material having a hardness less than that of the impeller on the interior surface of the wall of the case facing the impeller and in direct contact with the interior surface of the wall.
 3. The method of claim 2 wherein the step of inserting a material having a hardness less than that of the impeller on the interior surface of the wall of the case facing the impeller and in direct contact with the interior surface of the wall includes inserting the material on the axial compressor portion of the interior surface or the wall of the case.
 4. The method of claim 3 wherein the step of inserting the material on the axial compressor portion of the interior surface of the wall of the case includes holding the material in compression.
 5. The method of claim 4 wherein the step of inserting the material on the axial compressor portion of the case including serrating the surface of the material facing the impeller such that the surface is crenulated.
 6. The method of claim 2 wherein the step of inserting a material having a hardness less than that of the impeller on the interior surface of the wall of the case facing the impeller and in direct contact with the interior surface of the wall includes inserting the material on the axial compressor portion and continuing on the radial compressor portion of the interior surface of wall of the case.
 7. The method of claim 6 wherein the step of inserting the material on the axial compressor portion and continuing on the radial compressor portion of the interior surface of wall of the case includes holding the material in compression.
 8. The method of claim 6 wherein the step of inserting the material on the axial compressor portion and continuing on the radial compressor portion of the interior surface of wall of the case includes serrating the surface of the material facing the impeller such that the surface is crenulated.
 9. The method of claim 2 wherein the centrifugal compressor is a supercharger for supplying supercharged intake fluid to an engine.
 10. The method of claim 2 wherein the centrifugal compressor is a turbocharger for supplying supercharged intake fluid to an engine.
 11. A centrifugal compressor acting upon a compressible fluid, the centrifugal compressor comprising: a case having an axial compressor portion and a radial compressor portion; a rotatable impeller operable to compress the compressible fluid; an impeller shaft on which the impeller is supported, the impeller shaft being drivingly connectable to a power source; and a toroid insert on an interior surface or the wall of the case facing the impeller and being in direct contact with the interior surface.
 12. A centrifugal compressor as claimed in claim 11 where in the toroid insert is composed of a material having a material hardness less than that of the rotatable impeller.
 13. A centrifugal compressor as claimed in claim 12 wherein the toroid insert is composed of Teflon
 14. A centrifugal compressor as claimed in claim 13 wherein the toroid insert is placed in the axial compressor portion of the case.
 15. A centrifugal compressor as claimed in claim 14 wherein the toroid insert is adhesively bonded to the interior surface of the wall of the case in the axial compressor portion of the case.
 16. A centrifugal compressor as claimed in claim 14 wherein the toroid insert is held in axial compression.
 17. A centrifugal compressor as claimed in claim 14 wherein the toroid insert is held in radial compression.
 18. A centrifugal compressor as claimed in claim 17 wherein the toroid insert is held in place using at least one annular retaining member.
 19. A centrifugal compressor as claimed in claim 14 wherein the toroid insert is held in axial and radial compression.
 20. A centrifugal compressor as claimed in claim 14 wherein the surface of the toroid insert facing the impeller is smooth.
 21. A centrifugal compressor as claimed in claim 14 wherein the surface of the toroid insert facing the impeller is crenulated.
 22. A centrifugal compressor as claimed in claim 12 wherein the toroid insert is placed in axial compressor portion of the case and extending to the radial compressor portion of the case.
 23. A centrifugal compressor as claimed in claim 22 wherein the toroid insert is held in place using multiple torus retaining members and the multiple torus retaining members are recessed below the surface of the toroid insert that faces the impeller.
 24. A centrifugal compressor as claimed in claim 23 wherein the multiple torus retaining members have a tensile strength greater than that of the toroid insert.
 25. A centrifugal compressor as claimed in claim 23 wherein the toroid insert is adhesively bonded to the interior surface of the wall of the case in the axial compressor portion or the radial compressor portion of the case or both.
 26. A centrifugal compressor as claimed in claim 23 wherein the surface of the toroid insert facing the impeller is smooth.
 27. A centrifugal compressor as claimed in claim 23 wherein the surface of the toroid insert facing the impeller is crenulated.
 28. A centrifugal compressor as claimed in claim 22 wherein the surface of the toroid insert making contact with the casing contains locating tabs that protrude from the surface of the toroid insert and extend into the wall of the casing.
 29. A centrifugal compressor as claimed in claim 28 wherein the case has receiving holes on the interior surface of the wall of the case for receiving the locating tabs of the toroid insert.
 30. A centrifugal compressor as claimed in claim 23 wherein the toroid insert is adhesively bonded to the interior surface of the wall of the case in the axial compressor portion or the radial compressor portion of the case or both.
 31. A centrifugal compressor as claimed in claim 23 wherein the surface of the toroid insert facing the impeller is smooth.
 32. A centrifugal compressor as claimed in claim 23 wherein the surface of the toroid insert facing the impeller is crenulated.
 33. A centrifugal compressor as claimed in claim 28 wherein the locating tabs are torus rings that protrude from the surface of the toroid insert and extend into the wall of the casing.
 34. A centrifugal compressor as claimed in claim 33 wherein the case has channels on the interior surface of the wall of the case for receiving the torus rings.
 35. A centrifugal compressor as claimed in claim 22 wherein the toroid insert is held in place using multiple torus retaining members and the multiple torus retaining members are recessed below the surface of the toroid insert that faces the impeller and surface of the toroid insert making contact with the case contains locating tabs that protrude from the surface of the toroid insert and extend into the wall of the casing.
 36. A centrifugal compressor as claimed in claim 35 wherein the case has receiving holes on the interior surface of the wall of the case for receiving the locating tabs of the toroid insert.
 37. A centrifugal compressor as claimed in claim 35 wherein the toroid insert is adhesively bonded to the interior surface of the wall of the case in the axial compressor portion or the radial compressor portion of the case or both.
 38. A centrifugal compressor as claimed in claim 35 wherein the surface of the toroid insert facing the impeller is smooth.
 39. A centrifugal compressor as claimed in claim 35 wherein the surface of the toroid insert facing the impeller is crenulated.
 40. A centrifugal compressor as claimed in claim 22 wherein the toroid insert is held in place using multiple torus retaining members and the multiple torus retaining members are recessed below the surface of the toroid insert facing the impeller and surface of the toroid insert making contact with the case contains locating torus rings that protrude from the surface of the toroid insert and extend into the wall of the casing.
 41. A centrifugal compressor as claimed in claim 40 wherein the case has receiving channels on the interior surface of the wall of the case for receiving the locating rings of the toroid insert.
 43. A centrifugal compressor as claimed in claim 40 wherein the toroid insert is adhesively bonded to the interior surface of the wall of the case in the axial compressor portion or the radial compressor portion of the case or both.
 43. A centrifugal compressor as claimed in claim 40 wherein the surface of the toroid insert facing the impeller is smooth.
 44. A centrifugal compressor as claimed in claim 40 wherein the surface of the toroid insert facing the impeller is crenulated.
 45. A centrifugal compressor claimed in claim 12 wherein the centrifugal compressor is a supercharger for supplying supercharged intake fluid to an engine.
 46. A centrifugal compressor as claimed in claim 12 wherein the centrifugal compressor is a turbocharger for supplying supercharged intake fluid to an engine. 